1. Field of the Invention
The invention relates to a current detecting sensor, and more particularly to a current detecting sensor having three electrodes, applicable to a bio-sensor for measuring urine sugar. The invention relates also to a method of fabricating such a current detecting sensor.
2. Description of the Related Art
FIG. 1 illustrates a structure of a film constituting a sensitive section of a bio-sensor for measuring urine sugar. The film structure is comprised of a substrate 10, an electrode 12 formed on the substrate 10, and an organic film formed over the electrode 12 and the substrate 10.
The organic film is comprised of a first film 14 formed over the electrode 12 and the substrate 10, a second film 16 formed on the first film 14, and a third film 18 formed on the second film 16.
The third film 18 restricts transmission therethrough of a solution containing a material to be tested in order to ensure certain dynamic range. The second film 16 is a film to which glucose oxidase (GOD) acting as a catalyst is fixed. The first film 14 is a film preventing interfering materials from reaching the electrode 12. The first film 14 is comprised of a coupling material for enhancing adhesion between the first film 14 and the substrate 10, and a selectively transmissive film which does not allow selected interfering materials to pass therethrough.
The electrode 12 is comprised of an operation electrode 24, an opposing electrode 26, and a reference electrode 28 (see FIG. 2).
In the illustrated film structure, glucose to be measured reacts with enzyme by virtue of catalytic action of glucose oxidase in the second film 16, and as a result, there is generated hydrogen peroxide (H.sub.2 O.sub.2). The thus generated hydrogen peroxide is oxidized in the operation electrode 24 with the result of generation of oxidation current. The thus generated oxidation current is detected between the operation current 24 and an opposing electrode 26.
Glucose oxidase makes reaction in accordance with the following reaction formula (A). EQU Glucose+O.sub.2 Gluconic acid+H.sub.2 O.sub.2 (A)
The following reactions are made in the operation electrode 24 and the opposing electrode 26, respectively. EQU H.sub.2 O.sub.2 2H.sup.+ +O.sub.2 +2e- (B) EQU 2H.sup.+ +O.sub.2 /2+2e-H.sub.2 O (C)
The following reaction is made in an entire electrode system including the operation electrode 24 and the opposing electrode 26. EQU H.sub.2 O.sub.2 H.sub.2 O+O.sub.2 /2 (D)
When a reaction in the bio-sensor comes to an equilibrium state, a glucose concentration at time T can be measured, based on an amount of electrons (e-) in the above-mentioned reaction formulas, to flow into the electrode 12 per a unit time, which amount indicates a current generated at time T.
When hydrogen peroxide as an intermediate product is made to react on the electrode, it is necessary to apply a predetermined voltage across the operation electrode and the opposing electrode through a solution. However, there is generated a voltage drop in dependence on quality of a solution, and hence, a difference is made between a target voltage and an actual voltage. In order to prevent generation of such a difference, the bio-sensor is usually designed to include a reference electrode, which cooperates with the other two electrodes, namely, the operation electrode and the opposing electrode, to define a three electrodes structure.
In the bio-sensor having a three electrodes structure, a material of which a reference electrode is composed is required to have the following characteristics.
1. A reference electrode is difficult to be solved, and an oxidation film is unlikely to be generated.
2. A wide range of voltages can be checked without being interfered by a reaction to be made by a reference electrode itself.
3. Solvent and support salts are unlikely to be decomposed by a metal of which a reference electrode is composed.
4. It is possible to readily make a clean surface.
In order to meet with the above-mentioned requirements, a reference electrode may be composed of Pt, Au, Pd or C. If a reference electrode is used for detecting hydrogen peroxide, it would be important that a voltage at which hydrogen peroxide is detected is as small as possible, that a material of which a reference material is composed is not fused at the above-mentioned voltage, that a material of which a reference electrode is composed is unlikely to react with components basically contained in a solution, such as water and salts, namely, a base current is small, and that there is obtained high sensitivity for detection of hydrogen peroxide. Taking those into consideration, it is considered that Pt is best as a material of which a reference electrode is composed.
Platinum (Pt) has higher sensitivity to hydrogen peroxide by an order than that of gold (Au), and a voltage to be applied to a reference electrode composed of platinum is lower than a voltage to be applied to a reference electrode composed of gold. In addition, platinum has an advantage that it is less influenced by a substance such as ascorbic acid, which would be an interfering substance when urine is tested.
A current runs through an opposing electrode in a direction opposite to a direction in which a current runs through an operation electrode. Hence, an opposing electrode is composed of preferably of a material which has a small resistance, and which is not polarized under conditions for measuring hydrogen peroxide. In general, an opposing electrode is composed of the same material as a material of which an operation electrode is composed. That is, an opposing electrode is usually composed of platinum.
Both operation and opposing electrodes are formed on a substrate with a titanium layer being sandwiched therebetween, because a titanium layer has high adhesion with a substrate.
As mentioned earlier, a voltage of a reference electrode is determined in dependence on a reference electrode, and thus, a reference electrode is required to have the following characteristics.
1. A reaction which occurs at a surface of a reference electrode is a reversible one, and such a reaction responds to certain chemical species in accordance with Nernst's equilibrium potential equation.
2. A potential of a reference electrode is stable in spite of the lapse of time.
3. A potential of a reference electrode would quickly returns to an initial potential, even if a small current runs therethrough.
4. Even if a temperature varies, a reference electrode would have a fixed potential when a temperature varied to a certain temperature.
For instance, a reference electrode may be formed as a hydrogen electrode, a calomel electrode or an Ag/AgCl electrode. A reference electrode is usually formed as an Ag/AgCl electrode, because it can be readily fabricated, it has superior stability, and it can be handled with ease.
FIG. 2 is a cross-sectional view of a conventional sensor having a three electrodes structure. The illustrated sensor is comprised of an insulating substrate 10 composed of a material such as quartz and ceramic, an operation electrode 24 formed on the insulating substrate 10, an opposing electrode 26 formed on the insulating substrate 10, a reference electrode 28 formed on the insulating substrate 10 between the operation electrode 24 and the opposing electrode 26, and an organic film 30 covering the insulating substrate 10 and the electrodes 24, 26 and 30 therewith. The operation electrode 24 is comprised of a platinum layer and a titanium layer. The reference electrode 28 is comprised of a silver layer, a silver chloride layer, a platinum layer, and a titanium layer.
In the conventional sensor illustrated in FIG. 2, the silver chloride layer is gradually fused from an upper surface thereof in a thickness-wise direction while the sensor is in operation. Hence, there is generated a cavity in the reference electrode 28, resulting in that the organic film 30 is peeled off, and as an alternative, the organic film 30 is wrinkled. If the organic film 30 were peeled off because of a cavity having been generated in the reference electrode 28, the reference electrode 24 and the opposing electrode 26 are also influenced by the reference electrode 28 being peeled off.
The organic film 30 is deposited over the substrate 10 by spin coating. Since the organic film 30 is thin, the organic film 30 has poor coverage over edges of the electrodes 24, 26 and 28, and hence, is often cracked above the edges, resulting in that the organic film 30 begins to be peeled off from such a crack.
If the organic film 30 is peeled off due to fusion of the silver chloride film and poor coverage above the edges of the electrodes, sensitivity or response characteristic of the sensor would be degraded. FIG. 3A illustrates a relation between a sensor output and a glucose concentration in a sensor having stable sensitivity, whereas FIG. 3B illustrates the same in a sensor having unstable sensitivity. As illustrated in FIG. 3B, if sensitivity is unstable, a sensor output would be saturated at a low glucose concentration, resulting in that there cannot be obtained desired dynamic range.
In the conventional sensor, the silver chloride film constituting the reference electrode 28 is fused in a thickness-wise direction. The silver chloride film is usually designed to have a small thickness, specifically a thickness equal to or smaller than 1 .mu.m. A lifetime of the sensor terminates when the silver chloride film is all fused. Hence, the conventional sensor is accompanied with a problem of a short lifetime.
In addition, the reference electrode 28 is required to have a stable self-potential. However, the silver chloride film, the silver film, the platinum film and the titanium film are all exposed at a sidewall of the reference electrode 28, and hence, the unnecessary metal films, namely the silver film, the platinum film and the titanium film, prevent a self-potential from being stabilized.